PurpleMath.com
This is a website that I discovered last year while trying to find resources to help students to study for tests and quizzes. My students, and their parents, have found it to be very helpful. A student can search a topic and get many resources to help improve the understanding of the topic. There are lesson tutorials, learning forums, free online tutoring, quizzes, tests, worksheets, and homework guidelines. One of the features that I especially like is the “Study Skills Self-Survey”. It helps students to reflect on their study habits and introduces new strategies for students that may help to improve their learning. There are also links to other sites that may help students.

This website claims that it helps students to learn Algebra, but I teach pre-Algebra and I have found that the background knowledge and the misconceptions that are addressed in these lessons would help students in grades 7 through high school.